Conducting polymers including polyanilines, polypyrroles, and polythiophenes, have attracted attention as electrochemical actuators (See, e.g., K. Kaneto et al., U.S. Pat. No. 5,556,700; A. Mazzoldi et al., Mat. Sci. Eng. C6, 65 (1998).), since they are light-weight, low-cost, low-operating-voltage materials having the capability of generating high stress. Actuation is achieved by utilizing an electrochemical reaction to produce mechanical motion. Volume change of a conducting polymer during its redox process is thought to be achieved by electrolyte ion transport into and out of the polymer, solvent transport into and out of the polymer, polymer chain configuration changes, and electrostatic repulsion between polymer chains. Such actuators are expected to find applications in robots, artificial limbs, and other bio-mimetic devices, as examples.
The conductivity of the polymer plays an important role in determining polymer electroactivity and actuation. If the polymer is resistive, then the applied potential decreases with distance from the electrical contact. Applying a higher potential at the electrode to achieve a desired oxidation level further from the electrode may damage the material closer to the electrode. Moreover, oxygen reduction reactions further limit the electrochemical potentials in a resistive film (See, e.g., L. Bay et al., Proc. SPIE 4329, 54 (2001).). Thus, if the material is not highly conductive, a metal contact is required along the length of the polymer. The conductivity of conducting polymers used in electrochemical devices is typically less than 100 S/cm, with 300 S/cm being the highest conductivity reported for an actuator (See, e.g., M. Satoh et al., Synth. Met. 14, 289 (1986).), so a metal layer is required. Metal layers may corrode, react in the electrolyte, delaminate, or crack. In addition, use of a metal layer adds processing steps and expense to the production of actuators.
Aqueous electrolytes have a narrow electrochemical window, and some conducting polymers degrade in aqueous media (See, e.g., T. Kobayashi et al., Electroanalyt. Chem. 161, 419 (1984); and E. M. Genies and C. J. Tsintavis, J. Electroanalyt. Chem. 200, 127 (1986).). Moreover, aqueous electrolytes evaporate from unsealed containers thereof, rendering an electrochemical device involving their use inoperative. The use of non-aqueous electrolytes having wide electrochemical windows, high boiling points, and high ionic conductivity is advantageous.
Current quasi-solid actuators have a polymer or a gel containing a liquid electrolyte sandwiched between two conducting-polymer electrodes (See, e.g., Y. Min et al., Polym. Mat. Sci. Eng. 71, 713 (1994); and J. M. Sansinena et al., Chem. Commun. 2217 (1997)), where one electrode is the working electrode, and the other is the counter electrode; no reference electrode is needed. Such actuators generally function by bending, but actuators capable of linear motion have been reported (See, e.g., T. Lewis et al., Synth. Met 102, 1317 (1999); and U.S. Pat. No. 6,982,514 for “Electrochemical Devices Incorporating High-Conductivity Conjugated Polymers” which issued to W. Lu et al. on Jan. 3, 2006.).
Accordingly, it is an object of the present invention to provide a compact, two-electrode electrochemical device.
Another object of the invention is to provide electrochemical devices useful for actuation and energy storage.
Additional objects, advantages and novel features of the invention will be set forth, in part, in the description that follows, and, in part, will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.